As is generally known in the art, a hybrid electric vehicle uses both an internal combustion engine and a battery power source. The hybrid electric vehicle efficiently combines the torque of the internal combustion engine and the torque of a motor.
Hybrid electric vehicles may be categorized as either a hard type or a mild type according to a power sharing ratio between the engine and the motor. In the case of the mild type of hybrid electric vehicle (hereinafter referred to as a mild hybrid electric vehicle), a mild hybrid starter & generator (MHSG) configured to start the engine or generate electricity according to an output of the engine is used instead of an alternator. In the case of the hard type of hybrid electric vehicle, a driving motor for generating driving torque is used in addition to an integrated starter & generator (ISG) configured to start the engine or generate electricity.
The mild hybrid electric vehicle does not provide a driving mode in which torque of the MHSG is used for the main driving torque, but the MHSG may assist the torque of the engine according to the running state of the vehicle and may charge a battery (e.g., a 48 V battery) through regenerative braking. Accordingly, the fuel efficiency of the mild hybrid electric vehicle may be improved.
In general, the efficiency of a gasoline engine can be improved by operating the engine with a high compression ratio or by reducing the size of the engine. If the engine is operated with a high compression ratio or the size of the engine is reduced, however, knocking may occur and knocking may deteriorate the performance of the engine.
At the end of a combustion stroke, the pressure and temperature at an end-gas zone in a combustion chamber may increase and fuel may be self-ignited. Thus, knocking may occur due to the self-ignition of the fuel. If knocking occurs, the fuel is burned quickly and a piston may be damaged by vibrations. In addition, since combustion pressure is quickly lowered after reaching maximum pressure, the performance of the engine may be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.